Assuming you've checked all of your other settings (auto iso, exposure comp, etc.) to make sure something is not constraining the auto exposure, one thing it could be is that this shot includes a large expanse of what might be a wall painted white or a very light color. The auto exposure system assumes there is a normal distribution of tones in your scene, and sets the exposure such that the areas chosen for the exposure calculation (what areas they are depends on your auto exposure settings - multizone, center, etc.) should be exposed so the average works out to grey (like the old 18% grey reflectance cards). This might result in the overall image looking dark, if there are large areas of white in the scene.
--
Richard A. Novak